This invention relates to long-lasting phosphorescent glasses and glass-ceramics emitting a long persistent phosphorescence upon radiation of ultraviolet ray and visible light and so on, more particularly, to long-lasting phosphorescent glasses and glass-ceramics having a long-lasting phosphorescence and being capable of providing a rich variety of colors in the emitted phosphorescence.
Fluorescence is a phenomenon that a material emits visible ray when excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube and cathode ray tube (CRT) emit fluorescence. A material which emits fluorescence is called a phosphor. When light emitted by a phosphor lasts after stoppage of excitation for duration of time sufficient for the light to be perceived by the eye, i.e., about 0.1 second or longer, the light is called phosphorescence. A phosphor which has a long persistent phosphorescence lasting for several hours at room temperature is called a long-lasting phosphor or a light storage phosphor.
Known in the art of long-lasting phosphors are sulfides and oxides. ZnS:Cu is a typical sulfide type long-lasting phosphor which has been practically used for several decades. The ZnS:Cu long-lasting phosphor has the disadvantage that its after-glow lasts for a relatively short duration of time, i.e., about three hours at the longest. Further, this type of phosphor has the defect that a decomposition reaction of ZnS+H.sub.2 O.fwdarw.ZnO+H.sub.2 S takes place under the coexistence of ultraviolet ray contained in the sunlight and moisture contained in the air, making particles of the phosphor become black in color and, as a result, the after-glow characteristics significantly deteriorate within a relatively short period of time. For this reason, this type of phosphor has only limited applications such as a luminous watch and a night-time display of a location in a house.
On the other hand, there are reported, as typical oxide type long-lasting phosphors, m(Sr.sub.1-X Eu.sub.X).nAl.sub.2 O.sub.3.yB.sub.2 O.sub.3 disclosed in Chinese Patent No. CN1053807A, Zn.sub.1-X-Y Mg.sub.X.Mn.sub.Y and Sr.sub.I-X Al.sub.2 O.sub.4-X :Eu.sub.X,Ln.sub.Y disclosed in Japanese Patent Application Laid-open Publication No. Hei 8-127772, (Zn.sub.1-X-Y Mg.sub.X Mn.sub.Y)O.n(Ga.sub.1-Z Al.sub.Z).sub.2 O.sub.3 disclosed in Japanese Patent Application Laid-open Publication No. Hei 10-88126, and m(Sr.sub.1-a M.sup.1.sub.a)O.n(Mg.sub.1-b M.sup.2.sub.b)O.2(Si.sub.1-c Ge.sub.c)O.sub.2 : Eu.sub.x,Ln.sub.y disclosed in Japanese Patent Application Laid-open Publication No. Hei 9-194833. These long-lasting phosphors have high phosphorescence brightness and long lasting phosphorescence and also have good chemical durability and light-proof characteristics. These long-lasting phosphors are expected to have applications which, in addition to the conventional usage for luminous watches and night-time display of a location in a house, include disaster prevention marks, position indication marks for preventing danger and decorative articles. These long-lasting phosphors, however, are all made by the sintering process which involves complex manufacturing processes and according to which it is difficult to form a product of a desired shape. Besides, in these long-lasting phosphors, it is difficult to obtain a transparent product.
It is, therefore, an object of the invention to provide long-lasting phosphorescent glasses and glass-ceramics which, while realizing high phosphorescence brightness, long lasting phosphorescence, excellent chemical durability and light-proof characteristics, can be easily manufactured and easily formed to a desired shape and which provide both transparent and opaque products as desired.